1. Field of the Invention
The present invention relates to a split connecting rod that joins a piston pin and a crank pin, an engine including such a split connecting rod, and a vehicle including such an engine and split connecting rod.
2. Description of the Background Art
An engine for a car, motorcycle or other motorized vehicle uses a connecting rod that joins a piston pin and a crank pin. The connecting rod has a small end for rotatably holding a piston pin at one end of a rod body, and a large end for rotatably holding a crank pin at another end thereof. A piston pin opening into which the piston pin is inserted is formed in the small end, and a crank pin opening into which the crank pin is inserted is formed in the large end.
A split connecting rod has its large end split in advance between a cap and a rod along a split plane including the axial center of the crank pin opening. The large end with the crank pin opening is formed by joining the rod and the cap with bolts.
Such a split connecting rod requires accurate positioning of the cap and the rod upon assembly into a crankshaft, in order to maintain the crank pin opening of the large end in the shape of a perfect circle and cylinder.
Among attempted solutions for improving the positioning accuracy is a fracture technique. The fracture technique involves the integral formation of a large end, and subsequent fracture separation of the large end into a rod and a cap. Since the fracture planes of the rod and the fracture planes of the cap have fine irregularities, accurate positioning of the rod and the cap is achieved by mating the fracture planes of the rod and the cap.
U.S. Pat. No. 4,569,109 proposed the formation of fracture start grooves that extend linearly in the axial direction on the inner surface of a crank pin opening, in order to induce a fracture prior to the fracture separation of a large end into a rod and a cap.
According to the above-described fracture technique, the shape of fracture planes affects the accuracy of positioning the rod and the cap. In the presence of a plurality of fracture start points, in particular, different fracture planes may develop from the respective fracture start points. In this specification, the formation of different fracture planes will be called double cracking.
Joining of the respective ends of different fracture planes causes a difference in level at the junction portion, resulting in the formation of a sizable projection on the fracture planes. This projection causes a resultant crank pin opening to have a shape that is less than a perfect circle and cylinder upon assembly of the rod and the cap. Further, metal between different fracture planes may fall off as broken pieces, which can cause damage to the engine parts.
In recent years, a connecting rod having sufficient tolerance is needed for improving the tolerance of an engine. However, the use of a very tough material for a connecting rod to achieve improved tolerance often results in the formation of double cracks. For this reason, a carbon steel containing a large amount of carbon is usually used as the material of a connecting rod.
WO 99/06170 has proposed an apparatus for fracture separation for preventing the formation of such double cracks and broken pieces. The apparatus as disclosed in the above-mentioned patent document applies a load exclusively to one part of the inner surface of a crank pin opening of a large end before separation, to fracture and separate the large end with the load applied point as a start point.
The apparatus for fracture separation, however, has a complicated structure that requires precision in the jig used for the apparatus. This increases the equipment cost, while also presenting a burden of managing deterioration with time in the jig. As a result, the manufacturing cost of a split connecting rod is increased.